Solutions of acrylonitrile polymers in 1, 5-dimethylpyrrolidone-2



SOLUTIONS OF ACRYLONITREE POLYMERS IN LS-DIMETHYLPYRROLIDONE-Z George E. Ham, Decatur, Ala., assignor to The Chemstrand Corporation, Decatur, Ala., a corporation of Delaware No Drawing. Application October 9, 1953, Serial No. 385,273

7 Claims. (Cl. 260-302) This invention relates to a new method of preparing synthetic fibers from polymers of acrylonitrile. More particularly the invention relates to a new solvent for acrylonitrile polymers, and to completely miscible mixtures including acrylonitrile polymers, from which mixtures quality fibers can be extruded.

It is well-known that polymers of over 75 percent acrylonitrile are capable of being fabricated into high-strength fibers. The conventional technique for preparing fibers from these polymers involves the dissolution of the polymer in a suitable solvent and thereafter extruding the viscous solution so prepared through an orifice into a medium which removes the solvent and precipitates the acrylonitrile polymer in a continuous form. Many solvents have been proposed but many of them are impracticable due to various disadvantages possessed by them, such as excessive cost, poor color, tendency of the solutions to gel upon cooling, etc.

The primary purpose of this invention is to provide a new low cost solvent for the preparation of synthetic fibers. A further purpose of this invention is to provide a solvent which forms more stable mixtures or solutions of acrylonitrile polymers. A still further purpose of the invention is to provide a method of forming fibers of high tensile strength and desirable elongation.

It has previously been proposed (Houtz U. S. Patent No. 2,404,719) to use alpha-pyrrolidone as a solvent for acrylonitrile polymer solutions, and to employ the resulting solutions in the formation of fibers and films. However, it has been found substantially impossible and completely impracticable to use the resulting compositions in such a manner. What appears to be a solution of acrylonitrile polymers in alpha-pyrrolidone occurs only when the mixture is heated to 150 C., and slight cooling produces unusable gels. In commercial spinning operations it is essential that considerable quantities of the polymer solutions be prepared in advance of their use, and storage is, therefofe, an important step in fiber manufacturing operations. It is obviously impracticable to store large quantities of materials at temperatures above 150 C. It is not surprising that alpha-pyrrolidone is an apparent solvent only when maintained above 150 C. in view of the statement made in col. 6 of the Houtz patent that certain solvents disclosed therein dissolve or retain the polymer in clear solution only at elevated temperatures" and that below those temperatures the composition resembles a gel.

Heisenberg et al. U S. Patent No. 2,616,868 discloses that delta-methylpyrrolidone (also referred to as gammavalerolactam) has been proposed as a solvent for polyacrylonitrile. It has been found, however, that what appears to be a solution of polyacrylonitrile in delta-methylpyrrolidone occurs only at temperatures above 120 C. Slight cooling below that temperature produces a gel and for this reason the composition is not useful in spinning operations. I

N-methylpyrrolidone is also disclosed in the Heisenberg et al. patent as a solvent which has been proposed for Patent polyacrylonitrile. While not subject to the disadvantages possessed by alpha-pyrrolidone and S-methylpyrrolidone, this solvent cannot be used satisfactorily in the preparation of suitable fibers and films because of entirely different characteristics. It is well known that ideal spinning solutions are those wherein a solvent is used which will not dissolve acrylonitrile polymers to any appreciable extent at room temperatures, but which require mixing or slurrying of the solvent and polymer at these temperatures followed by gradual heating to eifect solution. In this manner the undesirable balling up phenomenon is avoided. Solvents such as N-methylpyrrolidone dissolve acrylonitrile polymers readily at room temperatures, but complete solution of all the polymer is not effected because of balling up, that is, the formation of a viscous film around lumps or balls of undissolved polymer. The film is not readily penetrated by additional solvent so that complete dissolution is possible only after prolonged and violent agitation.

Even when solutions of acrylonitrile polymers in N- methylpyrrolidone have been successfully prepared, it is not possible to prepare satisfactory fibers. therefrom by spinning into water due to the undesirable set-up or coagulation rate of the resulting solution in water. It has been found that the set-up rate of such solutions is much too rapid for the preparation of fibers having desirable physical properties, resulting in bulky, opaque, and voidfilled fibers.

Contrary to what would be expected in View of the prior knowledge regarding its homologs, it has now been discovered that 1,5-dimethylpyrrolidone-2 is an excellent solvent for acrylonitrile polymers and that the resulting stable solutions are particularly adapted to commercial spinning operations to produce fibers and filaments possessing superior physical characteristics. The 1,5-dimethylpyrrolidone-2, which may be readily obtained from levulinic acid, possesses several novel properties which make it outstanding as a spinning solvent, and in addition it possesses none of the disadvantages of its homologs.

The boiling point of 1,5-dimethylpyrrolidone-2 (189 C.) is desirable for wet spinning operations since it is dificult to protect workers from highly volatile solvents under such conditions as ordinarily employed. In addition to the resulting low vapor concentrations, the high boiling point contributes to eificient separation of solvent from water in recovery operations following spinning, and makes possible the preparation of bubble-free polymer solutions of high concentration at high temperatures using either batch or continuous techniques.

Unlike N-methylpyrrolidone, the solvent of this invention does not dissolve acrylonitrile polymers to any significant extent at room temperatures and the balling up phenomenon is avoided. Gradual heating of a mixture of 1,5-dimethylpyrrolidone-2 and the polymers to about 98 C. produces solutions of desirable viscosity ranges, and the solutions so prepared can be cooled to room temperatures and below without the formation of gels.

An important consideration in wet spinning 'is obtaining solvent and spinning bath compositions which give a proper coagulation rate or fiber set-up rate. Too rapid set-up results in bulky, opaque and void-filled fibers, whereas too slow set-up results in fiber breaks at the spinneret face or an uneconomically slow rate of spinning. The 1,S-dimethylpyrrolidone-Z is unique in that proper set-up rate may be obtained by spinning into pure water. Thus a wide latitude in choice of conditions necessary to yield fibers of good properties results.

While 1,5-dimethylpyrrolidone is an excellent solvent for copolymers of 70 percent or more of acrylonitrile and up to 30 percent of other polymerizable monomers,

' the invention is particularly useful with polymers of acrylonitrile containing at least 85 percent acrylonitrile and up to percent of another polymerizable monomer. The other monomer in the acrylonitrile copolymer may be vinyl acetate and other vinyl esters of monocarboxylic acids, methyl methacrylate and other alkyl esters of methacrylic acid, ethyl acrylate and other alkyl esters of acrylic acid, methacrylonitrile, vinylidene chloride, ethyl maleate and other alkyl esters of maleic acid, ethyl fumarate and other alkyl esters of fumaric acid, styrene and other vinyl substituted aromatic compounds, a-methylstyrene and other isopropenyl aromatic hydrocarbons, vinyl chloride and other vinyl halides, Z-Vinylpyridine, 2- methyl-S-vinylpyridine, and other vinyl substituted heterocyclic amines, and other polymerizable monomers capable of copolymerization with acrylonitrile.

The LS-dimethylpyrrolidone is also particularly useful as a solvent for processing fibers from blended compositions. Since many acrylonitrile polymers are not dyeable by conventional dyeing procedures, it has been proposed to blend them with polymers capable of reacting chemically with dyestuif, whereby the mixed compositions acquire dyeability such that the fibers have general purpose utility. Suitable blending agents are the polymeric compositions of of polymerizable monomers containing tertiary amino radicals or other radicals capable of being converted into tertiary amino groups subsequent to the polymerization. Thus copolymers of vinylpyridines, for example 2-vinylpyridine, the alkyl vinylpyridines, for example Z-methyl- S-vinylpyridine, the various vinylquinolines and alkyl substituted vinylquinolines, the various vinylpyrazines and alkyl substituted vinylpyrazines, the various vinyloxazoles and imidazoles including N-vinylimidazoles, and the vinylbenzimidazoles are useful. Similarly related allyl and methallyl derivatives of the above compounds are useful. The vinyl, allyl, and methallyl haloacetates can be reacted with secondary amines either before or after polymerization, and the polymers formed thereof blended with nondyeable acrylonitrile polymers to develop dye aflinity. These dyeable blending polymers may be homopolymers or they may be copolymers with any monomer polymerizable therewith, for example, acrylonitrile, styrene, butadiene, vinyl chloride, vinylidene chloride and vinyl acetate. In blending the polymers a substantial proportion of the fiber forming acrylonitrile polymer, for example 80 percent to 98 percent, should be used, depending upon the extent of dye receptivity desired and upon the proportion and relative efl ectiveness of the reactive monomer present. For example, a suitable blending polymer is one containing 10 to 70 percent by weight of acrylonitrile and 30 to 90 percent by weight of a vinylpyridine or an alkyl-substituted vinylpyridine. Suitable blends or blended compositions are those containing at least 75 percent by weight of acrylonitrile based on the total polymerized monomers in the blend. In general from two to ten percent of the total of all polymerized monomers present should be the reactive component.

All the polymeric materials described above, both form copolymers enable the practice of the invention more economically, permitting the utilization of continuous uninterrupted spinning and greatly minimizing fiber fractures and clogging of the spinnerets.

In the practice of this invention the polymers of acrylonitrile are used in finely divided form. Although massive polymers may be ground to desired particle size, preferably solvent-non-solvent polymerization procedures are employed in the preparation of the polymer. The subdivided states of the polymers obtained by spray drying the emulsions or by filtration and subsequent drying of the solid polymers enable them to be used directly. The finely divided polymer is mixed with l,S-dimethylpyrrolidone-Z in any type of mixing device, such as a dough mixer or a homogenizer. It is desirable to use a solution of as high a concentration of the polymer as possible, but the maximum concentration is dependent upon the molecular weight of the polymer.

To obtain fibers of optimum physical properties, polymers of molecular weights in excess of 25,000 are used, and when using such polymers it is only possible to dissolve from five to 35 percent in the 1,S-dimethylpyrrolidone-Z without exceeding practicable viscosity values. Although as low as five percent of the polymer can be used in spinning operations, such concentrations are undesirable because they necessitate the removal and recovery of too much solvent from the extruded solution, thereby increasing solvent recovery cost and reducing spinning speeds by reason of the longer periods required for coagulation. The concentration of the polymer in the solution is preferably between seven and 25 percent but will ultimately be determined by considering the desired physical properties of the fiber and the speed of spinning, which speed is a function of the concentration and viscosity of the polymer solution. The viscosity will depend upon the chemical composition and the molecular weight of the polymers. The optimum proportions can best be determined by selecting a uniform molecular weight polymer having good fiber forming properties and dissolving it in the smallest amount of the l,S-dimethylpyrrolidone-Z necessary to form a viscous solution capable of extrusion at convenient temperatures.

The fibers arespun by extruding the 1,5-dimethylpyrrolidone-2 solution of the acrylonitrile polymer through an orifice, or a spinneret having a plurality of orifices, into a medium which removes the solvent. The volume of the solution passed through the spinneret per unit of time must be constant in order to produce a fiber of uniform size. This is .best achieved by using a positively driven gear pump constructed of corrosion-resistant metals, such as stainlesststeel, and adapted to deliver a constant flow of solution regardless of minor changes in viscosity and regardless of the resistance offered by the spinneret. It is also desirable to pass the solution through one or more polymers and blends, are completely soluble in 1,5-dimethylpyrrolidone at 98 C., and solutions prepared by so heating the mixture of polymeric material and solvent are stable at room temperatures to give fluid solutions completely free of gel formations. The 1,5-dimethylpyrrolidone has excellent thermal stability and a high degree of resistance to hydrolysis even after prolonged heating.

In the practice of this invention, as it is in the preparation of all acrylonitrile fibers, the molecular weight of the polymer is of critical importance. The polymer should have a molecular weight in. excess of 10,000, and preferably in excess of 25,000. These molecular weights are determined by measuring the viscosity of the polymer when dissolved in a suitable solvent, such as dimethylformamide, in the manner Well-known to the art. It is also very desirable to use acrylonitrile copolymers which are substantially uniform throughout with respect to the chemical composition and physical structure. Such unifilters before reaching the spinneret in order towremove all possible traces of foreign matter and particles of incom pletely dissolved polymer. The polymer solution may be delivered to the gear pump by means of pressure applied by an inert gas to the liquid surface of the solution reservoir, which must be heated to maintain the solution fluid enough to pass through the conduits. The gear pump, filter devices and conduits to the spinneret are preferably heat insulatedand may be heated to maintain the body of solution in liquid state. The extruding operation should be conducted atelevated temperatures, but far enough below the boiling point of the solvent to prevent bubbles or other irregularities in the fiber.

The medium into which the solution is extruded and which removes the solvent may be either liquid or gaseous. The method involving the use of liquids is known as wet spinning; and any liquid which is a non-solvent for the acrylonitrilepolymer, but which either dissolves the 1,5- dimethylpyrrolidone-2, or converts it into soluble compounds, may be used. The solvent is leached out of the stream of polymer solutions, which first becomes a viscous stream and finally a solid filament. When a spinneret with a plurality of apertures is used the several streams of polymer converge and ultimately form a single fiber. The spin bath must necessarily be of suflicient size to permit the complete, or substantially complete, removal of the 1,S-dimethylpyrrolidone-Z. Obviously the rapidity of extrusion will afiiect the size of the spin bath, high speeds requiring much longer baths. The temperaures of the bath also affect the size, higher temperatures permitting more rapid diffusion of the l,S-dimethylpyrrolidone-Z from the fiber and enabling the use of shorter baths.

The use of 1,5-dimethylpyrrolidone-2 as a solvent for acrylonitrile copolymer is especially adapted to dry spinning operations, wherein air, steam, nitrogen or other gas, or mixtures of gases which are inert at the spin temperature, are used to remove the solvent. This method operates at higher temperatures; and the 1,5-dimethylpyrrolidone-2 is evaporated from the surface of the fiber. The maximum temperature to which the fibers can be subjected is usually the boiling point of the dimethylpyrrolidone, but higher temperatures may be attained by subjecting the operation to pressures higher than normal atmospheric pressure. The fiber may be heated by convection from the hot gaseous medium or by radiation from the walls of the spinning cell. Generally a combination of both convection and radiation is involved, but methods involving principally radiation are generally more efiicient and permit the operation with the wall temperature considerably in excess of the boiling point of the 1,S-dimethylpyrrolidone-Z. The evaporation of the dimethylpyrrolidone from the fiber surface and the speed of the fiber prevent the development of a temperature exceeding that at which the fiber is stable to decomposition. The dry spinning method is particularly useful at high rates of extrusion.

In general the methods of both wet and dry spinning commercially used are adaptable for spinning from 1,5- dimethylpyrrolidone-Z solutions, but special considerations may be involved due to the difierent chemical nature of 1,S-dimethylpyrrolidone-Z. Automatic machinery for spinning continuously, drying the thread if necessary, and winding it on suitable spools may be modified with the teaching of this specification. As in the case of most synthetic fibers, the fibers of acrylonitrile copolymers spun from 1,5-dimethylpyrrolidone-2 solutions may be stretched to develop optimum physical properties. If desired, part of the necessary stretching may be accomplished in the spinning medium by drawing the fiber out of the bath at a rate more rapid than the rate of extrusion.

The following examples in which parts, proportions and percentages are by Weight illustrate further the applications of the principles of the invention.

Example I Example 11 A mixture of one part of polyacrylonitrile and six parts of 1,5-dimethylpyrrolidone-2 was heated to 98 C. over a period of 33 minutes. A solution resulted which was stable on cooling and showed no evidence of gel formation. The solution precipitated in water to yield fibers and films.

Example 111 A spinning solution containing 12.5 percent solids was prepared by mixing 110 grams of a copolymer of 94 percent acrylonitrile and 6 percent vinyl acetate with 15 grams of a copolymer of 50 percent acrylonitrile and 50 percent 2-methyl-5-vinylpyridine, and dissolving the mixed polymers in 875 grams of 1,5-dimethylpyrrolidone at 98 C. The solution was cooled to 60 C. and extruded at this temperature through a 40-hole spinneret into water maintained at 60 C. The filaments were subjected to a jet stretch ratio of .47, a cascade stretch ratio of 3.5, and a relaxation ratio of 0.85, for an overall stretch ratio of 1.43. The resulting 5.5 denier fibers had an average tenacity of 1.5 grams per denier, elongation of 21%, knot tenacity of 1.34 grams per denier and knot elongation of 19% The fibers were clear and white.

I claim:

1. A new composition of matter comprising a homogeneous miscible mixture of from 65 to 95 percent by weight of 1,5-dimethylpyrrolidone-2 and from 5 to 35 percent of a polymer of which acrylonitrile is at least percent of the total polymerized monomer content of the polymer.

2. A new composition of matter comprising a homogeneous miscible mixture of from 65 to 95 percent by weight of 1,5-dimethylpyrrolidone-2 and from five to 35 percent of a copolymer of at least about 70 percent of acrylonitrile and up to about 30 percent of vinyl acetate.

3. A new composition of matter comprising a homogeneous miscible mixture of from 65 to 95 percent by weight of 1,5-dimethylpyrrolidone-2 and from five to 35 percent of a copolymer of at least about 70 percent of acrylonitrile and up to about 30 percent of a vinylpyridine.

4. A new composition of matter comprising a homogeneous miscible mixture of from 65 to 95 percent by weight of 1,5-dimethylpyrrolidone-2 and from five to 35 percent of polyacrylonitrile.

5. A new composition of matter comprising a homogeneous miscible mixture of from 65 to 95 percent by weight of 1,5-dimethylpyrrolidone-2 and from five to 35 percent of a blend of (A) a copolymer of at least percent acrylonitrile and up to 25 percent of another copolymerizable monomer and (B) a copolymer of from 30 to 90 percent of a compound of the group consisting of vinylpyridines and alkyl substituted vinylpyridines and from ten to 7 0 percent of another copolymerizable monomer.

6. A new composition of matter comprising a homogeneous miscible mixture of from 65 to 95 percent by weight of 1,5-dimethylpyrrolidone-2 and from five to 35 percent of a blend of (A) a copolymer of at least percent of acrylonitrile and up to 15 percent of another polymerizable monomer and (B) a copolymer of from 30 to percent of a compound of the group consisting of vinylpyridines and alkyl substituted vinylpyridines and from ten to 70 percent of acrylonitrile.

7. A new composition of matter comprising a homogeneous miscible mixture of from 65 to percent by weight of 1,5-dimethylpyrrolidone-2 and from five to 35 percent of a blend of (A) a copolymer of at least 75 percent by weight of acrylonitrile and up to 25 percent of vinyl acetate and (B) a copolymer of from ten to 70 percent by weight of acrylonitrile and from 30 to 90 percent of a compound of the group consisting of the vinylpyridines, and the alkyl substituted vinylpyridines.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,719 Houtz July 23, 1946 2,558,793 Stanin et a1. July 3, 1951 2,616,868 Heisenberg et a1. Nov. 4, 1952 

1. A NEW COMPOSITION OF MATTER COMPRISING A HOMOGENEOUS MISCIBLE MIXTURE OF FROM 65 TO 95 PERCENT BY WEIGHT OF 1,5-DIMETHYLPYRROLIDONE-2 AND FROM 5 TO 35 PERCENT OF A POLYMER OF WHICH ACRYLONITRILE IS AT LEAST 70 PERCENT OF THE TOTAL POLYMERIZED MONOMER CONTENT OF THE POLYMER. 